1. Field of the Invention
The present invention generally relates to a surface acoustic wave (SAW) filter for use as, for example, a bandpass filter, and to a communication apparatus including such a SAW filter. More particularly, the present invention relates to a SAW filter provided with a SAW filter portion and a SAW resonator electrically connected to the SAW filter portion on a piezoelectric substrate.
2. Description of the Related Art
The latest cellular telephone systems, along with an increase in the number of subscribers and a wider variety of services, use an increased frequency range. Accordingly, there is a strong demand for an increase in the pass bandwidth in SAW filters, which are widely used as bandpass filters in the RF stage of cellular telephones.
On the other hand, it is desired that the sensitivity of cellular telephones be uniform in all frequency channels. Accordingly, variations in the insertion loss in the pass band must be small.
Previously, longitudinally-coupled resonator mode SAW filters have been widely used as bandpass filters in the RF stage of cellular telephones. Japanese Unexamined Patent Application Publication No. 5-267990 discloses the structure of a longitudinally-coupled resonator mode SAW filter in which the pass bandwidth can be increased while reducing the insertion loss. In this SAW filter, three interdigital transducers (IDTs) are disposed on a 64xc2x0 Y-cut X-propagating LiNbO3 substrate having a large electromechanical coupling coefficient, and the center-to-center distance (pitch) of adjacent electrode fingers of the IDTs is xcex/4, to thereby increase the pass bandwidth.
However, in a longitudinally-coupled resonator mode SAW filter, when increasing the pass bandwidth, the uniformity of the insertion loss in the pass band is decreased.
In order to solve the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter including a piezoelectric substrate, a surface acoustic wave filter portion disposed on the piezoelectric substrate and having at least one interdigital transducer arranged to extend in a direction in which a surface acoustic wave propagates, and at least one surface acoustic wave resonator electrically connected in parallel with the surface acoustic wave filter portion such that the at least one surface acoustic wave resonator is disposed between an input terminal or an output terminal and the surface acoustic wave filter portion, wherein a resonance point or a sub-resonance point of the surface acoustic wave resonator is disposed in a pass band of the surface acoustic wave filter portion.
Preferred embodiments of the present invention have the advantage of providing a SAW filter with a balanced conversion function in which the uniformity of the insertion loss in the pass band is significantly increased. Also, the variation in the insertion loss in the pass band is minimized even if filtering characteristics are changed due to changes in temperature.
A SAW filter according to preferred embodiments of the present invention is advantageous since the deviation of the insertion loss in the pass band is minimized by utilizing the response at the resonance or sub-resonance point of the SAW resonator.
In the aforementioned SAW filter, the resonance or sub-resonance point of the SAW resonator may substantially coincide with the frequency at which the insertion loss in the pass band of the SAW filter portion is minimized. With this arrangement, the deviation of the insertion loss in the pass band can be more effectively reduced by utilizing the resonance or sub-resonance point.
In the aforementioned SAW filter, where the resonance point of the SAW filter is disposed on the pass band of the SAW filter portion, the Q value of the SAW resonator may be about 190 or less. With this arrangement, the deviation of the insertion loss in the pass band can be more effectively reduced without the influence of ripple caused by the SAW resonator.
In the aforementioned SAW filter, where the sub-resonance point is disposed in the pass band of the SAW filter portion, the SAW resonator may include an IDT and reflectors disposed at both sides of the IDT in a SAW-propagating direction, and the center-to-center distance of electrode fingers between the IDT and each of the reflectors preferably is approximately [(0.50 through 0.80)+0.50n] times (n is xe2x88x921, 0, 1, 2, and so on) a wavelength, which is determined by the electrode finger pitch of the IDT. With this arrangement, the deviation of the insertion loss in the pass band can be more effectively reduced.
A SAW filter according to preferred embodiments of the present invention may further include a pair of balanced signal terminals and an unbalanced signal terminal, thereby providing a balanced-to-unbalanced conversion function. In this case, it is possible to provide a SAW filter having a balanced-to-unbalanced conversion function in which the deviation of the insertion loss in the pass band is effectively minimized.
The present invention also provides a communication apparatus including a bandpass filter that is constituted by one of the SAW filters according to the preferred embodiments described above.
It is thus possible to obtain a communication apparatus having a bandpass filter provided with a balanced-to-unbalanced conversion function in which the uniformity of the insertion loss in the pass band is high, and the variation in the filtering characteristics due to temperature changes is small. Thus, such a communication apparatus has high transmission or reception characteristics.